In the 3GPP (3rd Generation Partnership Project), which is a standardization project of mobile communication systems, efficient use of a specific base station (e.g., a small cell base station) with a narrower coverage than that of a general base station (e.g., a macro cell base station) is being considered.
In the 3GPP, introduction of a dual connectivity scheme (Dual connectivity) is planned in Release 12 or thereafter (see Non Patent Literature 1). In the dual connectivity scheme, user terminal establishes connections with a plurality of base stations (a general base station and a specific base station) simultaneously. Since radio resources are assigned to the user terminal from each base station, an improvement in throughput is expected. The dual connectivity scheme is referred also to as inter-base station carrier aggregation (inter-eNB CA).
In the dual connectivity scheme, only one base station (hereafter referred to as a “master base station”) establishes an RRC connection with the user terminal among a plurality of base stations that establish connections with the user terminal. On the other hand, another base station among the plurality of base stations (hereafter referred to as a “secondary base station”) does not establish an RRC connection with the user terminal but provides the user terminal with additional radio resources.